Fix subsampling with formula-backed structural IDs

[anth-volk]

Fixes #498

Summary

• Use the full computed-variable export path inside Simulation.subsample() so internal dataset rebuilds preserve structural IDs such as formula-backed person_id.
• Add a synthetic regression test where the safe public export drops person_id__2022, but subsampling still succeeds.
• Add a towncrier bugfix fragment.

Tests

• uv run --frozen pytest tests/core/test_subsample_invalidates_cache.py -v
• uv run --frozen pytest tests/core/test_simulations.py::test__given_pseudo_input_in_dataset__then_input_dataframe_excludes_it -v
• uv run --frozen ruff check policyengine_core/simulations/simulation.py tests/core/test_subsample_invalidates_cache.py
• uv run --frozen ruff format --check policyengine_core/simulations/simulation.py tests/core/test_subsample_invalidates_cache.py